1. Field of the Invention
The present invention relates to a driving force controller for an electric motor vehicle for distributing driving forces individually to each wheel in accordance with values for driving force required for the vehicle and travelling conditions to drive each wheel independently by electric motors instead of a conventional motor.
2. Description of the Related Art
Recently, four-wheel drive (4WD) vehicles and electric cars have drawn special attention by the public.
Conventionally, a 4WD vehicle has one driving source and its driving force is transmitted to wheels mechanically through the intermediary of a transfer unit, a propeller shaft and front and rear wheel differentials, etc. The mechanism which performs a torque distribution function in the mechanical 4WD vehicle is the transfer unit, but generally it just distributes torques to front and rear wheels and the ratio of such distribution is limited by the mechanism. Furthermore, since such distribution is controlled by hydraulic pressure, in that type of 4WD in which the ratio may be controlled, accurate torque distribution cannot be expected.
Though capable of roughly distributing front and rear torques in accordance with shift of load caused by acceleration and other factors, the prior art control system referred to above cannot deal with a shift of load caused by centrifugal force when turning around a curve, especially when the load is shifted to an outer wheel.
In a selective type 4WD, for example, motive power is connected and disconnected at the middle of a drive shaft to switch between 4WD and 2WD, and is transmitted through a switching clutch in 4WDs, relative difference in rotation as between the front and rear wheels is not permitted, causing a tight corner braking phenomenon.
In a case of a center differential type 4WD, though relative rotation is allowed by a differential gear, only a fixed torque distribution ratio can be set due to the gear arrangement. Thus, when driving force cannot be adequately transmitted to the road surface due to reduced traction at either the front wheels or the rear wheels due to encountering a muddy spot, etc., the driving force transmitted to the other wheels is lowered automatically by the fixed distribution ratio and the overall driving force is thereby drastically lowered.
In a viscous coupling type 4WD, torque is transmitted from a member rotating at high speed to a member rotating at low speed when the difference in relative rotation between front and rear wheels becomes great. That is, normally it drives only with the front wheels, and torque is transmitted to the rear wheels only when the front wheels rotate idly. In a torque split type 4WD, a multi-plate clutch is used and transmission capacity is adjusted by hydraulic pressure to effect torque distribution. Though in many cases the hydraulic pressure is now controlled by a computer, active distribution cannot be expected if there is no relative difference in rotation.
FIG. 12 illustrates the limit for driving force at a tire.
Considering factors that determine driving force of a tire, the limit for driving force of a tire is determined by a balance between a limit for frictional force produced by a vertical load and the resultant driving force F including component Fx in the advancing direction and cornering force Fy in the lateral direction, as shown in FIG. 12. That is, when a load shift is caused by acceleration and turning and the load on a wheel is thereby changed, the limit for frictional force is changed accordingly. The limit for frictional force changes similarly when the road surface condition is snowy or muddy. Cornering force becomes great when a sharp turn is made and as a result, the limit for driving force in the advancing direction is lowered.
Therefore,
Limit of driving force Fx.max=(F.sup.2 -Fy.sup.2).sup.1/2
As described above, the conventional 4WD vehicles cannot produce an optimum distribution of driving forces since the limit of the driving force for each wheel changes with traveling conditions. For example, when a vehicle is to be accelerated while making a turn, load is applied to the outside thereof by centrifugal force, so that the grounding load on an inner wheel becomes smaller while the grounding load on an outer wheel becomes larger, creating a large difference between the limits for the driving forces applied at the outer and inner wheels. Although some conventional 4WD vehicles have a mechanism for restricting the differential between left and right wheels, such vehicles do not have active torque distribution and cannot control the difference between driving forces at the right and left wheels, so that only a relatively small driving force corresponding to the limit for driving force at an inner wheel may be applied. Cornering capability of the vehicle may be lowered and the vehicle may become unstable if the driving force exceeds that limit.
Moreover, when one wheel comes off or turns idly on a muddy spot in the conventional mechanical 4WD vehicle the driving force transmitted to the other wheels is drastically lowered since the right and left wheels are linked by a differential.
From the foregoing, the importance of controlling the four wheels independently, corresponding to their driving force limits, may be appreciated.
Much attention has been paid as of late to electric motor vehicles due to the problem of pollution by exhaust gas of conventional engine vehicles. Electric vehicles are now more and more regarded as important together with solar cars carrying solar batteries, i.e. clean energy, from the aspect of a global scale environmental problem. Because a solar car itself is fundamentally an electric car, development of an electric motor vehicle that can be widely put into practical use is strongly desired.
A vehicle with an electric motor typified by conventional electric cars normally has one motor and driving force is distributed by a differential gear unit and associated structure. Due to that, the ratio of driving forces distributed to right and left wheels remains constant.
The use of two motors to drive right and left rear wheels independently is disclosed in (Japanese Utility Model Application Laid-Open No. 55-138129). In this disclosure, when an accelerator pedal is depressed while travelling at speeds less than a set value, the motors are switched to four wheel driving.
When a vehicle is turning at high speed, however, the grounding load is shifted between right and left wheels since a large centrifugal force is generated on the vehicle body, so that the grounding load on the turning inner wheels is reduced and the grounding load on the outer wheels is increased as described above. At this time, because the conventional vehicle described above is designed so that torques to the right and left wheels are equalized by the differential gear, when the grounding load on the inner side wheels is reduced and the limit for driving force is thereby lowered, the limit for driving force at the outer side wheels is also lowered. As a result, slip limit speed is lowered.
Furthermore, because the conventional vehicle only drives four wheels in particular traveling conditions, even if the grounding load shifts during turning, driving forces cannot be effectively distributed responsive to the shift of the grounding load. That is, such a prior art vehicle fully utilizes the driving force of all motors only when the accelerator pedal is depressed while traveling at low speeds less than a set value and adopts the four wheel drive only temporarily.
The applicant named in the present application has previously proposed a driving force controller for an electric motor vehicle which eliminates the aforementioned problems (e.g. Japanese Patent Application No. 62-25736). This controller is provided with a means for detecting centrifugal force, a means for detecting the required driving force value and a computation controlling means for finding shift of loads from the centrifugal force to set driving forces for right and left wheels and for controlling electric motors for independently driving the right and left wheels. The controller drives the right and left wheels independently by driving forces which correspond to the grounding loads on the right and left wheels. Thus the driving forces that correspond to the grounding loads on the right and left wheels are distributed to achieve maximum utilization of the driving forces and acceleration is enhanced even during cornering, while preventing slip from occurring.
FIGS. 13A through 13D are drawings explaining various motions of an automobile. As shown in FIG. 13A, an automobile has a vertical motion, a yawing motion pivoting on the vertical motion, a longitudinal motion and a rolling motion pivoting on the longitudinal motion.
A vehicle which drives a plurality of wheels independently can control each driving force regardless of driving conditions, so that it can continue to generate driving forces without interruption caused by a given wheel. Accordingly, when trouble occurs at either the right or the left driving motor or wheel, while each wheel is being driven according to a predetermined driving force distribution ratio, the other driving motors keep generating the same driving force and a large difference between the driving forces at the right and left wheels is created, so that an undesirable yawing force (a vehicle turning force) is generated and the direction of the vehicle is changed as shown in FIG. 13B. If the vehicle is front wheel driving, a force is received at the steering wheel and a phenomenon whereby the steering force is changed in a manner similar to that experienced when a tire goes flat, as shown in FIG. 13C. The difference appears as a torque difference around a king pin as shown in FIG. 13d and a large manual steering force is required since one force is generated in the driving direction and another force is generated in the braking direction.
However, since the driving force controller of the electric motor vehicle in applicants' aforementioned application merely drives the right and left wheels in accordance with the grounding loads on the right and left wheels, the driving forces cannot be corrected even when the condition of the vehicle deviates from a predetermined range and a large difference is created between the driving forces at the right and left wheels and, accordingly, an undesirable yawing force is generated.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a driving force controller that suppresses changes in the behavior of the vehicle when traction 3 is lost or reduced at either the right or the left driving wheels.
It is another object of the present invention to provide a driving force controller that detects vehicle conditions to control driving force independently at each wheel so as not to deviate from a predetermined range.